View camera boundaries in user perspective?
Another option is to simply scale up the camera object. It doesn't effect the FOV at all. Select the camera, go to the properties panel and Camera > Display > Size.



Scale it up until the frustrum covers your painting area. You can see it from any angle. Alternatively you could parent a lamp with a square cone to match the camera frustrum.

Add a spot lamp and constrain it to the camera and copy the camera's Field Of View to lamp Size. You will have to scale the lamp's Y axis to fit aspect ratio.